The invention relates to polishing optical surfaces such as planar mirrors, spherical mirrors, aspherical mirrors, lenses and the like and more particularly to an apparatus for precisely polishing optical surfaces without manual intervention.
In the field of high precision optics used particularly in scientific optical instruments, telescopes and the like, precise flat surfaces as well as precise curved surfaces must be produced in optical elements to assure the highest possible instrument accuracy. The degree of accuracy required is relatively easily achieved for optical elements which are relatively small in size, however, for an optical element of the size typically used for a mirror in a reflector type telescope, the desired degree of accuracy is not easily obtained. Indeed, the desired optical characteristics for large mirrors used in reflector telescopes is achieved frequently only after years of tedious hand polishing. However, such manual correction is subject to being inaccurate and additionally suffers from being very time consuming. It also tends to produce small wave like variations in the surface contour.
Due to the time and expense of manual polishing, manufacturers have tried to automate the polishing operation with varying degrees of success. One such automated device simply automates the manual approach. Accordingly, measurements of the optical surface are made to locate areas where the contour deviates from that desired. Then, a mechanical polisher is located at the position where additional material must be removed. It is operated for a predetermined time and then moved to another position. These steps are repeated until all areas on the optical surface have been polished where additional material removal is desired. Thereafter, the surface contour is again measured and further polishing steps are performed as necessary. Mirrors made by this approach, however, have not proved to be as precise as desired so that manual polishing is required to make final correction of the surface contour.
Another automated approach involves moving a rotating polishing pad back and forth in a raster pattern across the surface being polished. In this method, the direction of movement of the rotating pad used to polish the surface changes abruptly at the edge of the surface being polished due to the back and forth movement of the pad. As a result, the polishing precision near the surface edge is not easily maintained. Additionally, where the surface being polished is either concave or convex in the direction of pad movement, the pad must be displaced in a vertical direction on each sweep of the pad across the surface. This added vertical pad movement further complicates the polishing process thereby making it more difficult to produce an optical surface having the desired surface contour.